This invention relates generally to bonding methods for use with materials such as glass, quartz, metal, ceramic and the like. More specifically, the invention relates to a method for bonding ring laser gyroscope components to the gyroscope body.
In one common form of compact ring laser gyroscope, a block comprising a glass, quartz, ceramic or similar material and having a low coefficient of thermal expansion (CTE) forms the body of the gyroscope. A number of components typically composed of glass or metal, such as electrodes, mirrors, and readout apparatus, are attached to the gyroscope body. Sealed passages in the body allow optical communication among the various components. The passages of the gyroscope body are filled with a lazing gas which lazes upon current being applied to the gyroscope.
A cathode and two anode components are used to create the beams of laser light traveling in opposing directions through the gyroscope body. The cathode and anodes may be composed of aluminum, steel, nickel or other metal which meets the design requirements for the gyroscope. The other components attached to the gyroscope may be glass mirrors, or may be glass-metal components which, for instance, allow adjustment of gyroscope mirror position to improve gyroscope performance.
The life and accuracy of the gyroscope is largely effected by the ability of the components to be properly bonded to the gyroscope body in such a way as to prevent escape of lazing gas, or contamination of the gyroscope passages with foreign gasses. In an ideal case, the seal should be hermetic, meaning that a negligible amount of gas is exchanged between the passages in the gyroscope body and the atmosphere during the life of the gyroscope. Thus the method used to seal the components to the gyroscope body is critical to the performance of the gyroscope.
The bonding method may also affect the operating range of the gyroscope, depending on the conditions under which the bonding materials degrade. Of particular concern are bonding materials having melting temperatures which limit the possible applications of the gyroscope (i.e. oil drilling, high speed/altitude aircraft, etc.) As another concern, the bonding of components to the gyroscope body ideally should not interfere with or alter previously completed processing steps, nor limit subsequent processing steps.
With these considerations in mind, numerous methods of bonding the components to the gyroscope body have been attempted, each with some measure of success. High temperature epoxy for example has been used as an effective material for glass-glass bonds. Indium or other soft metals have typically been popular for glass-metal bonds. Both have been effective in part because they are flexible enough to compensate for the differences in the CTE of the two materials being bonded. Other bonding methods, such as graded bonds and the use of glass frits, which attempt to match the CTE of the two materials to be bonded together, have also been successful.
Unfortunately, the epoxy and soft metal bonding techniques, due to the flexibility of the bond materials, tend to allow outgassing or fail to provide a bond capable of the types of pressures typically desired for high-end gyroscope devices. Indium in particular "squishes" out of the bonding area with repeated use of the device, eventually causing failure of the gyroscope. Neither epoxy or soft metal allow the gyroscope to be operated at high temperatures, since the limit of the gyroscopes range of operation is the melting temperature of the bonding material. In some cases an even lower limit is caused if the bonding material begins to degrade below its melting temperature. These same limits will effect the types of processing the gyroscope may undergo subsequent to formation of the bond.
Glass frits, which are used for bonding two identical materials, or materials with nearly identical CTE's together, require less cleaning and preparation of the bonding surfaces than required when forming indium seals. The use of glass frits is known to produce consistent and inexpensive hermetic seals. Unfortunately, the glass frit bonding process requires an elevated temperature which substantially limits the types of processing which can be done near the area of the bond prior to the bonding process. Furthermore, there is an inverse relationship between frit bonding temperature and the CTE for the frits used, which means that frits with low CTE, near that of the typical gyroscope body materials, have such high processing temperatures that they exceed the thermal limits of the gyroscope body. Thus, use of frits usually introduces a thermal mismatch into the gyroscope since a compromise must be made between bonding temperature and the CTE of the frit.
As a last point, gyroscope construction would be simpler if a single bonding material could be used to bond all components to the gyroscope body. Presently, individual bond techniques are used based on the type of component to be bonded, since no common bonding technique is known for all component types.